Elsevier

Thin Solid Films

Volume 520, Issue 7, 31 January 2012, Pages 3009-3012
Thin Solid Films

Complementary ring oscillator fabricated via direct laser-exposure and solution-processing of a single-layer organic film

https://doi.org/10.1016/j.tsf.2011.12.048Get rights and content

Abstract

A complementary ring oscillator is realized by exposing a solution-processed single-layer organic film to area-selective laser-light exposure and solution development. The pristine film comprises a blend of two organic semiconductors: p-type poly(3-hexylthiophene-2,5-diyl) (P3HT) and n-type [6,6]-phenyl C61 butyric acid methyl ester (PCBM). The exposure transforms PCBM into an insoluble form, and the subsequent development selectively removes the non-exposed PCBM while leaving exposed PCBM and P3HT intact. The 5-step ring oscillator exhibits a frequency of 10 mHz, a power delay product of 2.0 μJ, and an energy delay product of 22 μJs. Opportunities for performance improvements of the scalable fabrication technique are highlighted in an accompanying analysis.

Highlights

► Organic thin-film transistors prepared from solution-processed single-layer film. ► Complementary ring oscillator realized by selective area laser-light exposure. ► Light exposure step delimits film regions as p- or n-type. ► Fabrication technique compatible with large-area and low-cost production.

Introduction

Organic thin-film transistors (OTFTs) promise to contribute to the realization of paradigm-shifting electronic applications, such as conformable and ultra-thin emissive displays [1], [2], low-cost radio frequency identification tags [3], and electronic paper [4], due to attractive properties such as low-temperature processability from solution and mechanical flexibility. In order to qualify for these applications, it is important that the OTFTs can be combined into circuits, which simultaneously fulfill the requirements of low power consumption, high performance, and low-cost fabrication. From a power-consumption and performance perspective, it is often desirable to utilize the complementary metal oxide semiconductor (CMOS) technology, in which the electronic circuit comprises a combination of p-type and n-type transistors [5]. However, a challenge with the CMOS technology from an OTFT point-of-view is that it is difficult to deposit different types of organic semiconductors in close proximity over a large area in a reliable, scalable and low-cost manner [6]. A number of methods have been used to approach this problem including vacuum deposition through shadow masks [7], [8], [9], [10], [11], [12], [13], photolithography [14], micro-injector deposition [15] and inkjet printing [16]. While producing high performing OTFTs these methods are not ideal from the combined perspective of reliability, scalability and potential for low-cost production. Here, we demonstrate that it is possible to fabricate a relatively complex circuit, a CMOS ring oscillator, with a notably robust, scalable and straightforward method comprising solely solution processing and laser-light exposure. We also present an analysis of the static and dynamic response of the constituent CMOS inverters, and point out a viable path toward further improvements in oscillator performance.

Section snippets

Material and methods

[6], [6]-phenyl C61 butyric acid methyl ester (PCBM; 99.5%, Solenne b.v.) and poly(3-hexylthiophene-2,5-diyl) (P3HT; Sigma-Aldrich, regioregular, electronic grade, 99.995% trace metal basis, Mn = 30,000-60,000) were selected as the n-type and p-type semiconductors, respectively, and were separately dissolved in chlorobenzene (Sigma-Aldrich, anhydrous) in a 20 g/l concentration. A blend solution comprising PCBM and P3HT in a 5:1 mass ratio was spin-coated on a SiO2 gate dielectric (200 nm thickness)

Results

The non-exposed and developed transistors show p-type behavior, as only p-type P3HT remains in the active material, and a set of typical output data is presented in Fig. 2(a). The exposed and developed transistors are in contrast primarily n-type (see Fig. 2b), as dimerized PCBM dominates in the active channel; but a minor ambipolar character can be observed as a super-linearly increasing drain current with drain voltage at low gate voltages. The average mobility and threshold voltage for the

Discussion

Although the main aim of this study is to demonstrate that it is possible to realize advanced CMOS circuits by a straightforward and scalable approach comprising solution processing and direct laser light exposure, we wish to finish off by pointing out opportunities for future improvements in performance of the technique. The frequency of the ring oscillator can be estimated (considering the well-established equation for the gate delay [24]) as:f=IDSat2NCLVDDwhere IDSat is the average

Conclusions

In summary, we have applied a photolithographic technique on a single-layer organic blend film for the realization of a CMOS oscillation circuit, which when driven at a supply voltage of 50 V and an average supply current of 15 nA exhibits an oscillation frequency of 10 mHz, a power delay product of 2.0 μJ, and an energy delay product of 22 μJs. It is notable that the patterning of the organic blend film was effectuated without the use of a sacrificial photoresist material.

Acknowledgment

The authors acknowledge Kempestiftelserna, Carl Tryggers Stiftelse, and the Swedish Research Council (Vetenskapsrådet) for financial support. L.E. is a “Royal Swedish Academy of Sciences Research Fellow” supported by a grant from the Knut and Alice Wallenberg Foundation. Professor Robert Forchheimer at Linköping University is gratefully acknowledged for stimulating discussions and valuable input.

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